Editing a pdf document that has been docusigned

Try this. Instead of just downloading the document directly from DocuSign, click print as PDF and them you should be able to add more information to it. Unfortunately, the information already on it cannot be changed. 

Can a fully docusigned PDF be forwarded for further signatures? 

The idea is that once a PDF file has been digitally signed, either directly in Acrobat or via either the Adobe Sign service or DocuSign, it is no longer modifiable. The digital signature attests to the fact that the document is exactly what it was at the time it was signed. Thus, no further modifications to the PDF file!